TUI history insertion flushes stdout once per row, defeating write batching and synchronized output

What version of Codex CLI is running?

codex-cli 0.148.0-dev.1 (at 0.148.0-alpha.22 level + commits thru a998c7a1ce88a521d319a4f336e0a4bf36095637); reproduces on main @ a998c7a1ce88a521d319a4f336e0a4bf36095637

What subscription do you have?

ChatGPT Pro (20x)

Which model were you using?

Not model-specific; observed with gpt-5.6-sol

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What terminal emulator and version are you using (if applicable)?

Windows Terminal Canary 1.26.2182.0 with PowerShell 7.6.4. No terminal multiplexer. The mechanism is cross-platform; the visible impact is worst on Windows Terminal.

Codex doctor report

{
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      "details": {
        "ChatGPT base URL": "https://chatgpt.com/backend-api/ reachable (HTTP 403)",
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    "network.websocket_reachability": {
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      "summary": "Responses WebSocket handshake succeeded",
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        "DNS": "2 IPv4, 0 IPv6, first IPv4",
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}

What issue are you seeing?

Every multi-row history insertion in the TUI reaches the terminal as one write syscall per row instead of one batched write.

insert_history_hyperlink_lines_with_mode_and_wrap_policy writes each row with queue!(writer, Print("\r\n")) directly against the terminal backend (insert_history.rs#L139-L154, #L203-L216). The backend is CrosstermBackend<Stdout> over std::io::stdout() (tui.rs#L434), and Rust's Stdout is line-buffered when connected to a terminal, so each \r\n forces an immediate flush. queue! batching is silently defeated by the writer it queues into.

Effects:

1. Visible row-by-row rendering of large insertions. Each per-row write is an independently renderable chunk, so the terminal can paint between rows. This is the visible "dribble" in the transcript replay recordings on #37759 and #38839, but the mechanism applies to any large insertion — session resume, big tool output, scrollback repair.
2. Synchronized output (DECSET 2026) cannot hold the frame. Windows Terminal force-releases a synchronized update 100 ms after DECSET 2026 h. Thousands of per-row syscalls consume that budget, so large insertions escape the envelope partially painted. Any atomicity fix for #38839 that defers the clear into the sync update still repaints mid-replay unless the insertion is batched — single-write insertion is a precondition, not an optimization.
3. Thousands of independently paintable chunks per large insertion. Resize reflow may emit up to WINDOWS_TERMINAL_RESIZE_REFLOW_MAX_ROWS = 9_001 rows (resize_reflow_cap.rs#L20), i.e. ~9,000 write syscalls per reflow. The syscall overhead itself is minor — the CPU stall in #38839 is dominated by app-side reflow computation — but every flush is a point where the terminal can paint a partially applied insertion. It probably also exacerbates #38479 (conhost over RDP), where per-write processing costs more.

What steps can reproduce the bug?

1. Resume a session with a long transcript (or trigger resize reflow per #38839's steps) in inline mode.
2. Observe the insertion painting row-by-row rather than appearing as one frame. A syscall trace (ProcMon on Windows, strace -e write elsewhere) shows one write per history row during the insertion.

What is the expected behavior?

The complete insertion is prepared in memory and handed to the backend as a single write, making one insertion one renderable unit and letting synchronized updates fit their time budget.

Additional information

Issue introduced 0.11.0: the per-row write dates to PR #1710, which introduced the escape-sequence insertion path as queue!(std::io::stdout(), Print("\r\n")) in a per-line loop - every release since inserts history one flushed row at a time.

Candidate fix with a regression test: https://github.com/astral303/codex/compare/main...astral303:codex:fix/tui-single-write-history-insertion :

• the insertion path builds the complete ANSI batch in a Vec<u8> before touching the backend, removing per-row flushes while keeping terminal-state logic and the failure boundary unchanged
• a test asserts a multi-row insertion reaches the backend as exactly one write
• PR #1710 tests still pass

Interactive Windows Terminal regression coverage of the surrounding replay behavior (real 120x30 session, 1,500 long rows, resize cycles, visible-frame sampling) is in astral303/codex#7.